PROJECT SUMMARY The overall goal of this project is to prevent risk of second anterior cruciate ligament (ACL) injury in vulnerable populations, which consist of young, active men, women, girls and boys. Specifically, we propose to 1) stratify patients into risk groups for second ACL injury, 2) determine the effects of differential treatment on biomechanical measures of ACL injury risk and return to sport readiness after reconstruction (ACLR). Additionally, we will begin the development of a clinical algorithm that will classify patient risk for second ACL injury to facilitate the translation of findings from the laboratory into the clinic. Differential treatment will include a home only program, a standard clinical rehabilitation program, and a novel targeted neuromuscular training program designed to augment core and hip strength as well as promote optimum, low risk movement strategies in both lower extremities. Highly active men and women between the ages of 14 ? 24 years who have sustained an acute, isolated primary ACL injury will be recruited for study participation. As patients enter their final phase of rehabilitation after surgery they will undergo testing that includes self-reported function, performance of drop jumps, hop testing, strength assessment, and balance assessment. After testing, patients will be randomized into one of the three intervention groups for a six-week, twelve-session training program. After completion of training the patients will repeat the same battery of tests. To determine relative risk for second ACL injury, cluster-analysis statistical testing will be performed. Differences between risk-groups will be validated using the frontal plane hip and knee motions, which will be used as surrogates for ACL injury. Repeated measures ANOVA testing will be conducted to identify differences before and after testing to determine the effectiveness of each intervention program on clinical performance and biomechanical measures. Results from this proposed work will allow us to prospectively identify high risk patients who are the most appropriate recipients of enhanced treatment, including targeted neuromuscular training, which may reduce second ACL risk. Patients with lower-risk for second ACL injury may achieve optimum benefits from less intensive intervention. Thus, differential interventions may reduce the risk of a subsequent injury, enhance the quality of life for a significant cohort of young adults, and yield a more efficacious delivery of health care resources after ACLR.